ALLACTITE

Mn₇(AsO₄)₂(OH)₈                                      Monoclinic

	Figure 25-5. Crystal drawing of allactite from the Palmer Shaft area in Franklin. Drawing is from Palache (1935) who provided crystallographic data.

Allactite was first reported from Franklin and Sterling Hill by Palache (1935).

Description

Allactite from both Franklin and Sterling Hill occurs as superb 1-6 mm crystals. At Franklin, in general, they are bladed and tabular on {100} (Figures 25-5, 25-6, and 25-7); those found at Sterling Hill in 1981 are considerably thicker (Figure 25-8).

	Figure 25-6. Allactite crystals in parallel growth from Franklin. Field of view is 2.4 mm in maximum dimension.

The color varies; most specimens are brown, but the 1981 find at Sterling Hill was of distinctly brownish-red crystals. The luster is vitreous, inclining to adamantine. No physical measurements have been made. Optically, allactite is biaxial, negative, with small 2V, a = 1.761, b = 1.786, and g = 1.787; dispersion is very strong, r > v (Larsen, in Palache, 1935). The tabular habit is somewhat diagnostic; X-ray methods are best for verification.

Composition

Allactite is a manganese arsenate hydroxide mineral. Microprobe analyses (Dunn, 1983b) indicate that local material approaches end-member composition with but small amounts of Mg, Fe, and Ca substituting for Mn. Zinc substitution is limited, varying from 1.4 - 3.0 wt. %. Several analyses are presented in Table 25.

Occurrence and paragenesis

The diversity of assemblages evidenced by available specimens suggests that allactite may have been moderately common at Franklin but overlooked; it was reported from the Palmer Shaft by Palache (1935). It occurs primarily in carbonate-bearing vein assemblages and is associated with many minerals, among them pyroaurite, leucophoenicite, adelite, hodgkinsonite, barite, willemite, friedelite, and caryopilite. Franklin allactite crystals, in addition to forming clusters as shown in figures 25-6 and 25-7, also form druses on slickensides, some of which are coated with Mn-oxides and form thin crusts and veinlet fillings.

	Figure 25-7. Bladed allactite crystals in parallel growth from Franklin. Field of view is 2 mm in maximum dimension.		Figure 25-8. Stout allactite crystals in subparallel, slightly radial growth from Sterling Hill. Field of view is 1.5 mm in maximum dimension.

At Sterling Hill, allactite has been recovered in substantial quantities in recent years, but the crystal size is generally smaller than at Franklin, most being less than 2mm. Commonly associated minerals include kraisslite, sarkinite, willemite, rhodochrosite, barite, and calcite; less common associated minerals are fluorite, covellite, chlorophoenicite, friedelite, and secondary copper minerals.

The best of Sterling Hill allactite is that found in 1981 in divergent sprays, 3 mm in approximate size, and formed of thick brownish red crystals (Figure 25-8) (Dunn, 1983b). Allactite has also been found as colorless lath-like microcrystals coating thin vein-sidewall surfaces, as white splotchy patches on black oxidized vein surfaces, and as druses. Notable occurrences have been on the 1200 and 1500 levels.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 11, 2001